The present invention relates to aqueous compositions which are useful for creating protective coatings on metallic substrates. The protective coating may be readily removed once the substrate has been formed or otherwise shaped.
Many different goods are currently manufactured by drawing, pressing, bending or otherwise forming flat sheets of metal to the desired shape or configuration. In many cases, such metal sheets are finished with a decorative coating such as paint prior to forming. It will frequently be desirable to apply a temporary coating on top of the decorative coating in order to protect the decorative coating during forming, since forming may mar or otherwise damage the untreated decorative coating. Where the decorative finish is to be applied to the metal sheet after it is formed, temporary coating of the bare metal surface to protect it against corrosion or contamination will often be desirable. The development of temporary coatings which provide effective protection during forming, yet can be readily removed once forming is completed, thus would be of great value.
The present invention provides an aqueous composition comprising water, at least one polyvinyl alcohol, at least one surfactant and at least one corrosion inhibitor. In one embodiment of the invention, the aqueous composition has a mildly basic pH and is comprised of water, at least one polyvinyl alcohol, at least one surfactant (preferably a low foaming fluorinated, alkoxylated phenol or acetylenically unsaturated surfactant), at least two different corrosion inhibitors, and at least one coalescing solvent. A protective coating is obtained by applying the aforedescribed aqueous composition to a surface of a substrate (particularly a metallic substrate) and drying said aqueous composition. The dried coating protects the substrate surface, which may be bare or painted metal, during forming of the substrate. The protective coating is thereafter easily removed by a means such as peeling or rinsing with water.
The coating compositions of the present invention are aqueous in character. That is, water is used as the primary or predominate solvent or carrier for the other components of the coating composition. Although organic solvents, particularly alkanolamines and coalescing solvents, may additionally be present (as will be explained in more detail later), such solvents are utilized at relatively low concentrations (e.g., less than about 10 weight percent total, preferably less than about 6 weight percent total).
One or more polyvinyl alcohols are included in the aqueous composition. Such resins are commercially produced by the hydrolysis of polyvinyl acetate. The properties of the aqueous composition as well as the properties of the protective coating obtained using the aqueous compositions of this invention may be varied as desired by adjusting the degree of hydrolysis and the molecular weight of the polyvinyl alcohol. The degree of hydrolysis, for example, may suitably range from about 80% to about 100%, preferably 85% to 95%. Number average molecular weight may suitably range from about 10,000 to about 100,000. It is unnecessary to react or otherwise modify the polyvinyl alcohol prior to use. In preferred embodiments of the invention, both the polyvinyl alcohol and the aqueous composition in general are substantially free of plasticizer.
Polyvinyl alcohols useful in the invention may be readily obtained from commercial sources, including Air Products and Chemicals Inc. (which sells polyvinyl alcohols under the trademark AIRVOL) and DuPont (which sells polyvinyl alcohols under the trademark ELVANOL). While the concentration of the polyvinyl alcohol in the aqueous composition is not believed to be particularly critical, typically concentrations of from about 2 to about 50 (preferably, about 5 to about 20) percent by weight (calculated on a solids basis) are utilized.
Other water-soluble and/or water-dispersable polymeric resins may optionally also be present in the aqueous composition, including, for example, acrylic resins, polyethylenimines, polyethylene oxides, polyvinyl pyrrolidones, polyacrylamides and the like. Polyvinyl alcohol is generally the predominate polymeric resin in the composition, however (i.e., at least 50% by weight of the polymeric resin component of the composition is comprised of polyvinyl alcohol).
One or more surfactants (surface active agents) are also incorporated in the aqueous composition to provide satisfactory spreading and leveling of the aqueous composition on the surface of the substrate being coated. The surfactant(s) promote wetting of the substrate surface and also help to stabilize the aqueous composition against phase separation during storage. Preferably, the surfactants selected for use are low foaming in order to minimize the formation of bubbles in the protective coating. Fluorinated surfactants, alkoxylated phenol surfactants, and acetylenically unsaturated non-ionic surfactants have been found to be particularly suitable for use. Suitable fluorinated surfactants may be of the non-ionic, anionic, or amphoteric type, including combinations or mixtures thereof. Nonionic fluorinated surfactants include perfluoroalkyl polyethers and fluoroaliphatic polymeric esters such as those, for example, described in U.S. Pat. No. 5,747,234, incorporated herein by reference in its entirety. Illustrative anionic fluorinated surfactants include fluoroaliphatic sulfates and fluoroaliphatic sulfonates such as those for example, described in U.S. Pat. No. 5,616,273, incorporated herein by reference in its entirety. Suitable amphoteric fluorinated surfactants include surfactants containing fluoroaliphatic groups, carboxyl groups and amine and/or amide groups such as those described in U.S. Pat. No. 4,069,244 (incorporated herein by reference in its entirety). Amine perfluoroalkyl sulfonates may also be used. Fluorinated surfactants useful in the present invention may be readily obtained from a number of commercial sources including Ciba-Geigy (under the trademark LODYNE), Minnesota Mining and Manufacturing (under the trademark FLUORAD), Elf Atochem (under the trademark FORAFAC) and E. I. duPont de Nemours (under the trademark ZONYL). Suitable acetylenically unsaturated non-ionic surfactants include surfactants containing at least one hydroxylated or alkoxylated acetylenically unsaturated hydrophobic moiety. Preferably, such surfactants contain two hydrophobic groups connected by an acetylenic linkage with hydroxyl as alkoxy (preferably, ethoxy) groups pendant near the acetylenic linkage. Each hydrophobic group connected with the acetylenic linkage preferably contains from 4 to about 10 carbon atoms. Surfactants of this type are available commercially from Air Products and Chemical Company under the trademark SURFYNOL.
Suitable alkoxylated phenol surfactants include those materials prepared by reacting a phenol such as an alkyl phenol (e.g., nonylphenol) with one or more epoxides such as ethylene oxide and/or propylene oxide. Surfactants of this type are available commercially from Hoechst, for example.
The precise amount of surfactant(s) used in the aqueous compositions of the present invention is not thought to be critical; the optimum concentration for a particular application may be readily determined by routine experimentation. Typically, however, surfactant levels of from about 0.05 to about 0.6 percent by weight (based on the weight of the aqueous composition) will be effective.
One or more corrosion inhibitors (substances capable of inhibiting the corrosion of a metallic substrate surface) are also present in the aqueous composition. Although any of such materials known in the surface treatment field may be employed, it has been found that the use of one or more alkanolamines in combination with one or more corrosion inhibitors other than alkanolamines is advantageous.
Suitable alkanolamines include those compounds containing at least one nitrogen atom and at least one hydroxyl group substituted on an alkyl group including, for example, amino-alkanols, monoalkyl amino-alkanols, dialkyl amino-alkanols and mixtures thereof. Illustrative alkanolamines include dimethylethanolamine, diethylethanolamine, dimethylpropanolamine, 2-amino-2-methyl-1-propanol, 2-dimethylamino-2-methyl-1-propanol, mono-, di- and triethanolamine, N,N,N1,N1,-tetrakis-(2-hydroxyethyl)-ethylene diamine and the like. In addition to functioning as corrosion inhibitors (e.g., protecting the surface of a ferrous substrate treated with the aqueous compositions against rusting), the alkanolamines also adjust the pH of the aqueous composition and act as solvents.
Other suitable corrosion inhibitors include nitrates, nitrites, borates, phosphates, silicates, borated amines, carboxylated amines, organic acids and their esters, metal salts and anhydrides (e.g., N-oleyl-sarcosine, sorbitan mono-oleate, lead naphthenate, dodecenyl-succinic acid and its partial esters and amides, 4-nonylphenoxy acetic acid), primary, secondary and tertiary aliphatic and cycloaliphatic amines and amine and zirconium salts of organic and inorganic acids (e.g. amine salts of nitrous acid, chromic acid, carbonic acid, carbamic acid, aliphatic carboxylic acids, aryl-substituted aliphatic carboxylic acids, and substituted or unsubstituted benzoic acids), nitrogen-containing heterocyclic compounds (e.g., thiadiazoles, substituted imidazolines, diazoles, triazoles, oxazolidines, oxazolines), quinolines, quinones, anthraquinones, propyl gallate, barium dinonyl naphthalene sulfanate, ester and amide derivatives of alkenyl succinic anhydrides or acids, benzothiazolylthio-substituted succinic acids or anhydrides, dithiocarbamates, dithiophosphates, and amine salts of alkyl acid phosphates, polyamine fatty acid salts and the like. Especially preferred corrosion inhibitors include alkanolamines, adducts of carboxylic acids and amines, nitrogen-containing heterocyclics, and mixtures thereof.
In one preferred embodiment of the invention, one or more adducts of carboxylic acids and amines are used in combination with one or more nitrogen-containing heterocycles (especially oxazolidines).
Sufficient amounts of one or more corrosion inhibitors are incorporated in the aqueous composition so as to increase the corrosion resistance of the temporary protective coating formed therefrom, as compared to an analogous coating which does not contain corrosion inhibitor. The optimum amount of corrosion inhibitor(s) will vary depending upon the corrosion inhibitor(s) selected for use, among other factors, but may be readily determined without undue experimentation. Typically, the aqueous composition contains from about 0.5 to about 7 weight percent corrosion inhibitor in total. In one embodiment of the invention, a combination comprised of from about 0.1 to about 5 weight percent alkanolamine(s) and from about 0.1 to about 5 weight percent of at least one additional corrosion inhibitor other than an alkanolamine corrosion inhibitor (preferably a nitrogen containing corrosion inhibitor, more preferably a corrosion inhibitor selected from the group consisting of nitrogen-containing heterocyclics and adducts of carboxylic acids and amines) is employed. In another embodiment of the invention, from about 1 to about 5 weight percent of one or more nitrogen-containing heterocycles (preferably, oxazolidines) and about 0.5 to about 2.5 weight percent of one or more adducts of carboxylic acids and amines are used in combination.
The aqueous composition preferably includes at least about 0.2 weight percent of one or more coalescing solvents which are miscible with or soluble in water. Coalescing solvent concentrations of up to about 10 percent by weight will typically be effective, although preferably no more than about 5 percent by weight coalescing solvent is present. Any of the solvents known in the coatings field to assist in the formation of a continuous smooth film may be utilized, including, for example, alcohols, glycols (e.g., propylene glycol), glycol oligomers (e.g., tripropylene glycol), polyalkylene glycols (e.g., polypropylene glycol), polyols containing three or more hydroxyl groups (e.g., glycerin), and ethers and esters thereof (e.g., glycol ethers, glycol esters and polyol esters). Monoalkyl ethers of propylene glycol, ethylene glycol, dipropylene glycol and diethylene glycol, where the alkyl group contains from 1 to 6 carbon atoms, may be used, for example.
The viscosity and other rheological properties of the aqueous compositions may be varied as desired by the addition of one or more thickeners (also referred to as rheology modifiers). Although the optimum viscosity may vary considerably depending upon the particular end-use application for the aqueous composition, typically the viscosity will be in the range of from about 100 cps to about 5000 cps (preferably, about 500 cps to about 3000 cps) at 25xc2x0 C. as measured using a Brookfield digital viscometer (RV spindle #4, 20 rpm). While any of the substances known to be capable of increasing the viscosity or otherwise modifying the rheological characteristics of an aqueous composition may be employed, particularly preferred thickeners include polymeric thickeners. Suitable polymeric thickeners include acrylic copolymers such as styrene-acrylic copolymers and urethane-acrylic copolymers. Acrylic polymer thickeners are well known in the field and are readily available from commercial sources such as Rohm and Haas (under the trademarks ACUSOL and RHOPLEX), Johnson Wax (under the trademark JONWAX) and CCP Polymers (under the trademark CHEMPOL). Thickeners based on urethane block copolymers are also particularly preferred for use in the present invention, especially hydrophobically modified urethane thickeners. Such substances are polyurethanes which contain both hydrophobic groups (e.g., at least two hydrocarbon groups having at least seven carbon atoms each) and hydrophilic groups (sufficient to ensure solubility in water or water/water-miscible solvent blends). The preparation of hydrophobically modified urethane thickeners is described in U.S. Pat. Nos. 4,079,028 and 5,281,654, incorporated herein by reference in their entirety. Hydrophobically modified urethane thickeners are also available from commercial sources under the trademarks ACRYSOL RM-825, ACRYSOL RM-8W, ACRYSOL RM-12W, ACRYSOL SCT-275, RHEOLATE 225 and TAFIGEL PUR60.
Generally speaking, thickener concentrations of from about 0.5 to about 5 weight percent are utilized, although lesser or greater amounts may be present depending upon the thickener(s) selected, the rheological properties and concentrations of the other components of the aqueous compositions (particularly the polyvinyl alcohol), and the extent of thickening desired. In one embodiment of the invention, however, no thickener is utilized.
The aqueous composition preferably is neutral to basic, with mildly basic aqueous compositions being particularly preferred. The composition may be rendered basic by the incorporation of ammonia, alkanolamines, and other amines, for example. Such additives also help neutralize the acidity often associated with the polyvinyl alcohol component. The pH is desirably in the range of from about 6 to about 11 (more desirably, from about 8 to about 10 to avoid any degradation or attack of aluminum or painted surfaces). Other ingredients in addition to the aforementioned components may also be present in the aqueous compositions of the invention such as, for example, flow modifiers, surface control additives, colorants (dyes, pigments), stabilizers, defoaming agents, and the like.
The aqueous composition is made by conventional means, typically including a step of mixing the components of the aqueous composition at atmospheric pressure so as to form a substantially homogeneous mixture. In a preferred embodiment of the invention, the components are selected such that the aqueous composition remains stable during storage (i.e., does not phase separate). The components may be heated during mixing, if necessary.
The typical and preferred concentration ranges of the components described hereinabove are stated in reference to the levels in the aqueous composition when it is to be actually applied to the substrate surface.
The aqueous composition is applied to the surface of a substrate by one of a variety of techniques. Preferred techniques include brushing or spraying of the aqueous composition onto the surface, although dipping, calendaring, rolling or other methods may also be utilized. If phase separation has occurred, it will generally be desirable to thoroughly mix the aqueous composition before applying it to the substrate surface. In preferred embodiments, the resulting wet coating of aqueous composition is from about 5 to about 25 mils thick, more preferably about 8 to about 20 mils thick.
The coating of aqueous composition is then permitted to dry, normally at ambient temperatures and pressures although drying can be accelerated if so desired by heating using hot air, infrared heat or the like. However, curing (chemical reaction) of the components of the aqueous compostion is not required in order to form a coating capable of providing good protection during forming of the substrate to which the coating is applied. In one embodiment of the invention, the aqueous composition does not contain any components capable of reacting with other components under conditions normally encountered during drying. Once dry, the aqueous composition forms a protective film coating (typically, from about 0.3 mils to about 5 mils thick) which is impact resistant, drawable, formable, and easily removed. The term xe2x80x9cformablexe2x80x9d is defined as the ability of a coated sheet of metal to be bent without creating substantial cracks or voids in the film. The term xe2x80x9cdrawablexe2x80x9d is defined as the ability to stamp a coated sheet of metal into a curved three-dimensional shape without substantially breaking the sheet and without causing significant damage to the coated sheet of metal. For many typical applications, the protective coatings provided by the aqueous compositions of the present invention show remarkable improvements in the afore-stated properties.
Removal of the dried coating is easily effected by either peeling off the coating or contacting the coated substrate with water, preferably an aqueous alkaline solution. An advantage of the present invention is that the water or aqueous alkaline solution need not be heated substantially above room temperature to accomplish effective and rapid removal of the protective coating. xe2x80x9cContactingxe2x80x9d means spraying, flooding, dipping (immersion) and other such methods.